Thanks for getting back Nick.
Perhaps common is a bit strong 
but there were a few along similar lines -
As you probably noticed I'm working a souped-up version of your annotate node.
As there is the possibility to directly type a hex value into the colour picker, I though it best to validate this. However although it marks an error on the node, the input element that gets the error class is a hidden object.
The code you shared should be a least a help to know which picker has the incorrect value, thanks.
